The present invention relates generally to a hydraulic retarder control system.
In the field of large-sized vehicles such as trucks, buses and the like, as disclosed in U.S. Pat. Nos. 4,262,781 and 4,711,328, there is known a vehicle which includes a hydraulic retarder system. The hydraulic retarder system, when the vehicle runs down along the downward path, generates a braking torque to thereby reduce the high speed of the vehicle and prevent a friction brake device from fading due to the rise in temperature, so as to improve the safety of the vehicle and the durability of friction members. The hydraulic retarder system includes a rotor fixable to a rotary shaft such as a propeller shaft or the like which is rotated together with a wheel, and a stator fixed to the body of the vehicle in such a manner that it is not rotatable. In the hydraulic retarder system, the rotor is connected and fixed to the rotary shaft by a clutch device, operation liquid is stirred by the rotor, and there is generated a brake torque due to the friction loss of the operation liquid and also due to the loss of collision with the stator.
However, since the brake torque to be produced by the hydraulic retarder system, when the pressure of the operation liquid within the hydraulic retarder is constant, is obtained according to the number of revolutions of the rotary shaft, that is, the running speed of the vehicle and thus the brake torque obtainable in the low speed running state of the vehicle is small, there is a possibility that a driver unfamiliar with the hydraulic retarder system can drive his or her car to ram a car ahead when the driver is going to stop the vehicle or when driving at the low running speed. Also, when the driver continues to run the vehicle at the low speeds of 30 to 50 Km/h in a long downward slope such as a hill road, the speed reduction obtained by the hydraulic retarder is small, resulting in an inferior operation feeling.